Climate change is widening the world’s gap between the haves and have-nots, worsening economic inequality between rich and poor countries, according to a new analysis by Stanford scientists.

The difference between the economic output of the world’s cool wealthy nations and hot struggling nations is 25 percent larger today than it would have been without global warming, according to researchers Noah Diffenbaugh and Marshall Burke.

“Our results show that most of the poorest countries on Earth are considerably poorer than they would have been without global warming,” said climate scientist Diffenbaugh. “At the same time, the majority of rich countries are richer than they would have been.”

Much of the debate over climate change focuses on future risks of flooding and other disasters. But this analysis, published Monday in the Proceedings of the National Academy of Sciences, shows the price that many countries have already paid.

Previous work found that during warm years, northern nations like Norway, Sweden and Iceland get an economic boost, while tropical and subtropical nations like India, Nigeria and Brazil suffer from slowed productivity.

The new study takes a much broader and longer look at the impact of climate change. Although economic inequality between countries has decreased in recent decades, the gap would have narrowed faster without the problem, according to the research.

For instance, India’s GDP — the aggregate value of the economy’s goods and services – is about 30 percent lower today than it would have been if there hadn’t been global warming, they found. It’s 29 percent lower in Nigeria and 25 percent lower in Brazil.

Norway’s GDP is 34 percent higher than in a world without climate change. It’s 32 percent higher in Canada and 9.5 percent higher in Great Britain.

Agriculture explains much of the difference. In cool regions, warming lengthens the growing season and allows a greater diversity of crop species. In warm regions, heat reduces yield of commodity crops like corn, soybeans and wheat.

But there are other contributors. Cool nations need to spend less money on energy to stay warm, while warm nations spend more money to stay cool.

The research combines two approaches: A statistical analysis of the impact of temperature fluctuations on economic growth and 20 climate models created by research centers around the world and used by the Intergovernmental Panel on Climate Change, which advises the world’s governments under the auspices of the United Nations.

The team calculated that what each country’s economic output might have been had temperatures not warmed.

For any particular nation, the annual impact is small, said Diffenbaugh.

“But it is like a retirement account,” he said. “Small differences in what’s contributed 30, 40 or 50 years ago compounds, and creates a big differences in what is available when you retire.”

While the biggest emitters enjoy on average about 10 percent higher per capita GDP today than they would have in a world without warming, the lowest emitters have been dragged down by about 25 percent.

Such a drag “is on par with the decline in economic output seen in the U.S. during the Great Depression,” said Burke, Stanford assistant professor of Earth system science.

“It’s a huge loss compared to where these countries would have been otherwise,” he said.

Lisa M. Krieger is a science writer at The Mercury News, covering research, scientific policy and environmental news from Stanford University, the University of California, NASA-Ames, U.S. Geological Survey and other Bay Area-based research facilities. Lisa also contributes to the Videography team. She graduated from Duke University with a degree in biology. Outside of work, she enjoys photography, backpacking, swimming and bird-watching.